The neurotransmitter acetylcholine binds to two types of cholinergic receptors: the ionotropic family of nicotinic receptors and the metabotropic family of muscarinic receptors. Muscarinic receptors belong to the large superfamily of plasma membrane-bound G protein coupled receptors (GPCRs). To date, five subtypes of muscarinic receptors (M1-M5) have been cloned and sequenced from a variety of species, and show a remarkably high degree of homology across species and receptor subtype. These M1-M5 muscarinic receptors are predominantly expressed within the parasympathetic nervous system which exerts excitatory and inhibitory control over the central and peripheral tissues and participate in a number of physiologic functions, including heart rate, arousal, cognition, sensory processing, and motor control.
Muscarinic agonists such as muscarine and pilocarpine, and antagonists, such as atropine have been known for over a century, but little progress has been made in the discovery of receptor subtype-selective compounds, thereby making it difficult to assign specific functions to the individual receptors. See, e.g., DeLapp, N. et al., “Therapeutic Opportunities for Muscarinic Receptors in the Central Nervous System,” J. Med. Chem., 43(23), pp. 4333-4353 (2000); Hulme, E. C. et al., “Muscarinic Receptor Subtypes,” Ann. Rev. Pharmacol. Toxicol., 30, pp. 633-673 (1990); Caulfield, M. P. et al., “Muscarinic Receptors—Characterization, Coupling, and Function,” Pharmacol. Ther., 58, pp. 319-379 (1993); Caulfield, M. P. et al., International Union of Pharmacology. XVII. Classification of Muscarinic Acetylcholine Receptors,” Pharmacol. Rev., 50, pp. 279-290 (1998), the disclosures of which are incorporated herein by reference.
The Muscarinic family of receptors is the target of a large number of pharmacological agents used for various diseases, including leading drugs for COPD, asthma, urinary incontinence, glaucoma, Alzheimer's (AchE inhibitors), and Pain.
Pain can be roughly divided into three different types: acute, inflammatory, and neuropathic. Acute pain serves an important protective function in keeping the organism safe from stimuli that may produce tissue damage. Severe thermal, mechanical, or chemical inputs have the potential to cause severe damage to the organism if unheeded. Acute pain serves to quickly remove the individual from the damaging environment. Acute pain by its very nature generally is short lasting and intense. Inflammatory pain on the other had may last for much longer periods of time and it's intensity is more graded. Inflammation may occur for many reasons including tissue damage, autoimmune response, and pathogen invasion. Inflammatory pain is mediated by an “inflammatory soup” that consists of substance P, histamines, acid, prostaglandin, bradykinin, CGRP, cytokines, ATP, and neurotransmitter release. The third class of pain is neuropathic and involves nerve damage that results in reorganization of neuronal proteins and circuits yielding a pathologic “sensitized” state that can produce chronic pain lasting for years. This type of pain provides no adaptive benefit and is particularly difficult to treat with existing therapies.
Pain, particularly neuropathic and intractable pain is a large unmet medical need. Millions of individuals suffer from severe pain that is not well controlled by current therapeutics. The current drugs used to treat pain include NSAIDS, COX2 inhibitors, opioids, tricyclic antidepressants, and anticonvulsants. Neuropathic pain has been particularly difficult to treat as it does not respond well to opiods until high doses are reached. Gabapentin is currently the favored therapeutic for the treatment of neuropathic pain although it works in only 60% of patients where it shows modest efficacy. The drug is however very safe and side effects are generally tolerable although sedation is an issue at higher doses.
Despite the large therapeutic value of this family, cholinergic drugs are limited by the lack of selectivity of these agents, with significant activation of the parasympathetic autonomous system and elevated incidence of adverse effects. The molecular cloning of the muscarinic receptors and the identification of the physiological role of specific isoforms using knock-out mice, has recently delineated novel opportunities for selective muscarinic ligands, and has helped to define the selectivity profile that is required for enhanced efficacy and reduced side effects.
There is a need for modulators of muscarinic receptors M1-M5. There is also a need for methods for treating muscarinic receptor-mediated diseases.
There is also a need for modulators of muscarinic receptors that are selective as to subtypes M1-M5.